Angle Grinder Material Removal Speed Comparison

[Wamsutta]

4.5 inch disc spinning at 11,000 R.P.M.
7.0 inch disc spinning at 6500 R.P.M.

-Both grinders are grinding the same material.
-Both grinders are using a grinding disc, not a cutting disc.
-Both grinding discs are the exact same brand and composition.
-Both grinders are using the outside edge of the disc, not the face of the disc.

Which do you think will remove material faster?

 

[d.mcfarland]

The larger one in this case I bet.

Chances are it has a more powerful motor which allows more pressure to be used.

The smaller one in your example actually would have a faster speed at the point of contact, however, it probably bogs down a little more than the larger/more powerful grinder.

 

[KnurledNut]

Formula incomplete.
No motor sizes.

 

[Lucid Moments]

Assuming the 7" has enough motor to keep it spinning then the 7" should remove more material. It will have a larger contact patch, and I don't feel like doing the match this morning but the edge speed should be faster even though the RPMs are lower.

 

[mike93lx]

Speed (fps, at no load) is essentially the same at the edge of those disc's, assuming they are actually 4.5" and 7"...just slightly faster on the 4.5

The bigger deciding factor will be motor power and how fast the disc actually runs when under load

 

[bwringer]

Let's do some mathing:

Circumferences:
14.14"
22.00"

Edge speeds:
155,540 ipm (inches per minute) (2,592.33 ips) at 11,000 rpm
143,000 ipm (2,383.33 ips) at 6,500

So, ASSSSSSuming the speeds stay constant under load, which they do not in the real world, the smaller grinder at higher RPM will remove material at a slightly higher rate.

The Torque Test Channel on the Tube of You has some excellent real-world testing of angle grinders (and much else) that demonstrates that the rated RPMs, torques, etc. printed on the tool's cardboard box are pretty much wild fantasy.

 

[GaryM909]

The weight of the 7" grinder will help with removing metal.

 

[bwringer]

The weight of the 7" grinder will help with removing metal.

Oh, sure, **** up our theoretical discussion with real-world considerations...


But yeah, contact pressure is of course a fundamental factor; 7" grinders have a LOT more power, so they can sustain more RPM, and a LOT more weight.

Theoretically, the size of the contact patch doesn't matter in terms of pure friction (it's CoF and force). However, in the real world the larger contact patch of the 7" disc would help the abrasive last longer and generate less concentrated heat in the remaining material and abrasive.

In the Torque Test Channel's testing, they've found that watts (work) is a pretty darn good indicator of material removal rates. The more watts your motor can put into the work, the more material gets removed per minute.

The bigger motor wins, and the diameter of the disc at the other end isn't all that relevant.

 

[RTM]

But will using the larger grinder wear out the operator sooner, as he struggles to control that beast? Thus leading to more breaks, beer, and trips to the ER?

 

[tarbellb]

My real world usage says 7"

my body and safety still grab a 5"

 

[Steve_P]

My real world usage says 7"

my body and safety still grab a 5"

My world and body also. My old B&D 7" grinder is a beast. I don't work on heavy equipment, so I don't have too many reasons to use it (it was my dad's); I can't remember the last time I used it, but I'd bet 15+ years.

 

[bwringer]

Another example of the gap between theory and practice is the effects of disc wear.

The OP's question assumed using only the outer edge, not the face. Assuming similar rates of wear (which is almost certainly a bad assumption; the 7" disc is likely thicker) the 4.5" and 7" discs are going to lose diameter at different rates, and edge speed on one will deteriorate faster than on the other. The 4.5" disc will probably deteriorate faster and require changing more often... all else being equal, which it isn't.

Urgh... I'm not doing the math. So anyway Wammy, what was the question? Or did we just get roped into doing homework for your kid?

 

[KnurledNut]

My old B&D 7" grinder is a beast.

Wildcat? Beast mode entered the chat.

 

[908Jim]

Let's do some mathing:

Circumferences:
14.14"
22.00"

Edge speeds:
155,540 ipm (inches per minute) (2,592.33 ips) at 11,000 rpm
143,000 ipm (2,383.33 ips) at 6,500

So, ASSSSSSuming the speeds stay constant under load, which they do not in the real world, the smaller grinder at higher RPM will remove material at a slightly higher rate.

The Torque Test Channel on the Tube of You has some excellent real-world testing of angle grinders (and much else) that demonstrates that the rated RPMs, torques, etc. printed on the tool's cardboard box are pretty much wild fantasy.

Great analysis for a new disk here, but I'd also like to add that the 4.5" disc would wear roughly 2x the rate of the 7" disk because of the higher rotational speed. As the disk wears, the surface speed will drop off faster than on the 7" grinder due to the diameter reducing, so you'd see more noticeable performance degradation with extended use.

In practice, 4.5" all day because they're just nicer to work with in my opinion.

 

[M635_Guy]

My brushless Milwuakee 2888 at 11A vs. the brushed Hercules at 13A is a no-contest win for the Hercules with the same cutting wheel.

Amps make work, so while I really appreciate bwringer doing the math (my head was already hurting before I got there), I think the more-grumpy thing is going to be more effective.

I saw a very interesting video from Jason at Fireball Tool on how scoring and working down a channel took longer but saved a ton of the disc.

 

[ObnoxiousFumes]

I saw a very interesting video from Jason at Fireball Tool on how scoring and working down a channel took longer but saved a ton of the disc.

Would you mind posting a link to that video if you have it? I’m having trouble picturing what you mean.

 

[mike93lx]

Would you mind posting a link to that video if you have it? I’m having trouble picturing what you mean.

 

[rlitman]

...The bigger motor wins, and the diameter of the disc at the other end isn't all that relevant.

For the most part, yes, but if you run a 4.5" disc in an 7" grinder, it is going to cut very slowly, so surface speed does mater for certain.

As for amps or watts or horsepower or some other stand-in for power as a measurement of material removal rate, I would agree that that's probably true. Which also means that my 14" electric cut-off grinder isn't all that much more capable than a 7" grinder, aside from cut depth. And why a Stihl TS420 is going to cut WAY faster, due to the gas engine providing more horsepower than you can get out of a standard outlet.

 

[ObnoxiousFumes]

Ohh ok, cutting discs not grinding, interesting. Thanks @mike93lx!
Seems like he burned through that disc way to quick on the first test, that didn’t seem right. I use a combination of both techniques I guess and my 5” discs last way longer than that...
Anyway, this is sorta off topic.

 

[tarbellb]

Technique outweighs damn near everything else

I had guys on my crew who'd blow through 2x the disc as me and cut half the material

the 5" x .045" disc is the sweet spot
minimal cutting thickness, large diameter, strong enough to give the user confidence (quality disc only)

 

[Sumboodie]

Put the 7" disc on the 5" grinder.

 

[mike93lx]

Put the 7" disc on the 5" grinder.

Guard's off anyway, so you might as well get some **** done

 

[rlitman]

Technique outweighs damn near everything else

I had guys on my crew who'd blow through 2x the disc as me and cut half the material

the 5" x .045" disc is the sweet spot
minimal cutting thickness, large diameter, strong enough to give the user confidence (quality disc only)

So the OP's question was to measure material removal rate from the circumference. However, linear cutting speed depends greatly on wheel thickness (kerf), which puts 5" grinders at an advantage, because they're pulling as many amps as 7" grinders, but with thinner wheels.

As for running 7" wheels on a 5" grinder, I for one am not going to overspeed my wheels.

 

[M635_Guy]

Ohh ok, cutting discs not grinding, interesting. Thanks @mike93lx!
Seems like he burned through that disc way to quick on the first test, that didn’t seem right. I use a combination of both techniques I guess and my 5” discs last way longer than that...
Anyway, this is sorta off topic.

Technique outweighs damn near everything else

I'm often a little off-topic or tangent, but in this case tarbellb is saying what I was trying to say.

 

[ObnoxiousFumes]

As for running 7" wheels on a 5" grinder, I for one am not going to overspeed my wheels.

Me neither. Funny thing, it was just last night my grandpa was just telling me about some guy that did that back when he was working on the paving crew.
I also don’t take the guard off except on rare occasions where it won’t fit where I’m working. The gouge in my middle finger knuckle is why.
Well, that and the fact that I’ve heard of blades grenading every now and then. Here I go off topic again...

the 5" x .045" disc is the sweet spot

Now I’m curious, have to go measure my Walter’s, they’re a lot thicker than the cheap ones I used to have.

 

[Dakotadadv8]

I have the Makita corded with the 4.5 works fine grinding, cutting rebar and metal, and cement blocks. Still going strong 15 years get the right blades and not cheap.

 

[bwringer]

FWIW, not that anyone cares by this point... the OP specified grinding discs, not cutting discs, using only the edge, and material removal as the metric.

 

[strutaeng]

The 7" is a heavier machine and has more power, so stock removal is almost always going to be faster in my experience. No comparison.

I've got several 4.5"s, a 5, 6, and a 7". The 7" is a beast for stock removal, just a bit heavier. Use the weight to your advantage grinding with the 7". 5" and 6" are good combination of speed and balance.

 

[seber]

Larger contact patch and higher force favors the 7" by a very large amount.

 

[Sumboodie]

So the OP's question was to measure material removal rate from the circumference. However, linear cutting speed depends greatly on wheel thickness (kerf), which puts 5" grinders at an advantage, because they're pulling as many amps as 7" grinders, but with thinner wheels.

As for running 7" wheels on a 5" grinder, I for one am not going to overspeed my wheels.

Many 7" wheels are rated high enough. Id say most, but I don't know for sure.

Edit...

It's 6" wheels I use.

 

[PoorUB]

4.5 inch disc spinning at 11,000 R.P.M.
7.0 inch disc spinning at 6500 R.P.M.

The bigger deciding factor will be motor power and how fast the disc actually runs when under load

For the argument, I would assume this is the loaded RPM.

 

[ItsNemo]

Let me answer that for you:

Was exactly what I wanted to figure out in this video, material removal rate with various grinder sizes.

 

[LopezBart]

Guard's off anyway, so you might as well get some **** done

I found grinding down the guard ( carefully or with another grinder ) so that it just prevents the wheel from touching the surface when the grinder is set down keeps my hands safe, but I don't get annoyed by the guard hitting the work.

 

[mike93lx]

For the argument, I would assume this is the loaded RPM.

Just trying to at least note reality